1. Field of the Invention
The present invention relates to an optical pickup device for recording data in a disc-shaped data storage medium as represented by an optical disc and/or reproducing data from the data storage medium, and an optical disc device incorporated with the optical pickup device.
2. Description of the Background Art
Disc-shaped recording/reproducing media such as CD and DVD for recording or reproducing information with use of a light beam, flexible discs for recording or reproducing information using magnetism, and MO and MD for recording or reproducing information using a light beam and magnetism have been widely spread. In particular, owning to a large data capacity and a low cost, optical discs such as CD and DVD are widely spread as recording media for use in a computer, or recording media for recording video/music. In recent years, there is a demand for an optical disc having a larger data capacity, as the amount of data to be recorded in an optical disc is increased.
It is necessary to increase the recording density of data by reducing the size of a light spot to be defined by irradiation of light onto an optical disc in recording data in the optical disc and/or reproducing data from the optical disc in order to increase the data capacity of the optical disc. The size of a light spot can be reduced by using a light source for emitting laser light of a short wavelength, and increasing the numerical aperture (NA) of an objective lens in an optical pickup device. In an optical pickup device for emitting laser light on DVD, a light source having a wavelength of 650 nm, and an objective lens having a numerical aperture (NA) of 0.6 are used. Nowadays, with use of a blue laser having a wavelength of 405 nm, and an objective lens having NA of 0.85, BD (Blu-ray Disc) having a recording density five times of the recording density of a currently available DVD has been spread.
BD has multiple recording layers to increase the recording capacity, and a first recording layer and a second recording layer are formed with a predetermined interval in the disc thickness direction. A thickness variation of a cover layer of an optical disc through which laser light is transmitted may greatly affect the spherical aberration of an optical system having a large NA. Accordingly, a large spherical aberration may occur in switching the position of a light spot to be condensed from one recording layer to another recording layer. In order to correct the spherical aberration, a spherical aberration correcting mechanism is loaded in the optical pickup device, wherein a movable lens is provided on an optical path from a light source to an objective lens in an optical pickup device to correct the spherical aberration by driving the movable lens in the optical axis direction and adjusting the position of the movable lens.
FIG. 15 is a perspective view showing a schematic arrangement of a conventional spherical aberration correcting mechanism. Referring to FIG. 15, the spherical aberration correcting mechanism 200 includes a lens holder 220 for holding a movable lens 210, a main shaft 230 and an auxiliary shaft 240 for movably supporting the lens holder 220 in the optical axis direction (the arrow direction 200A and the arrow direction 200B) of the movable lens 210, a screw shaft 250 arranged substantially in parallel to the main shaft 230 and formed with a helical groove, a stepping motor 260 with a rotating shaft to be directly connected to the screw shaft 250, a rack member 270 which is mounted on the lens holder 220 in engagement with the helical groove of the screw shaft 250, and operable to convert a rotating movement of the screw shaft 250 into a rectilinear movement of the lens holder 220, and a photosensor 280 for detecting the position of a light blocking plate 220a mounted on the lens holder 220.
The spherical aberration correcting mechanism 200 is operable to move the lens holder 220 to an intended position along the main shaft 230 and the auxiliary shaft 240 in the optical axis direction (the arrow direction 200A and the arrow direction 200B) of the movable lens 210 by rotatably driving the stepping motor 260. Specifically, the spherical aberration correcting mechanism 200 is operable to correct a spherical aberration by rotatably driving the stepping motor 260 depending on a spherical aberration amount resulting from a thickness variation of a cover layer, or switching the position of a light spot between recording layers; and moving the movable lens 210 (see e.g. Japanese Unexamined Patent Publication Nos. 2003-45068A and 2003-91847A).
In the spherical aberration correcting mechanism 200, the lens holder 220 is guided by shaft engagement to drivingly move the lens holder 220 in the optical axis direction (the arrow direction 200A and the arrow direction 200B). Specifically, the main shaft 230 is engaged in two main shaft bearings (not shown) of the lens holder 220, and the auxiliary shaft 240 is engaged in a U-shaped auxiliary bearing 221 to restrain rotation of the lens holder 220 about an axis of the main shaft 230. In this arrangement, the lens holder 220 is guided in the optical axis direction (the arrow direction 200A and the arrow direction 200B).
As described above, in the conventional spherical aberration correcting mechanism 200, the lens holder 220 is guided by shaft engagement. Accordingly, engagement play is necessary between the main shaft 230 and the main shaft bearings to smoothly guide the lens holder 220 along the main shaft 230. Because of the engagement play, a contact point between the two main shaft bearings and the main shaft 230 is not uniquely defined, and the contact point is changed, as the lens holder 220 is driven. As a result, the lens holder 220 may be tilted with respect to the main shaft 230, and control of the lens holder 220 may be unstable due to shake of the movable lens 210.
In the guide arrangement using shaft engagement, it is necessary to assemble the parts after the two guide shafts (the main shaft 230 and the auxiliary shaft 240) are mounted on the lens holder 220 by shaft engagement, in loading the spherical aberration correcting mechanism 200 in the main body of the optical pickup device. This may deteriorate the assembling performance, and increase the number of assembling steps, with the result that the production cost may be increased.